I'm with Jovidan.
Although to each his own, I'd stay for three extra days in a heartbeat.
The deluxe resorts are great and all if money isn't an issue, but for me I'm at WDW to experience the attractions and fun OUTSIDE of the resorts. If I just wanted to stay at a fancy hotel there are plenty near my home that make even the most beautiful WDW property look like a shanty town.
And think of the money you will save - if you get a discount code, you can book Pop Century for like $55/night. You'd pay for 7 days what you'd likely pay for only two nights at a deluxe. I'd much rather have that cash to actively spend on fancy meals (Brown Derby, World Showcase, etc.) and stuff in the parks than a room that will remain empty unless I'm asleep.
It really comes down to where your priorities lie - yes, deluxes have bigger rooms and more "services", but are you really going to use it? Especially with the new "Magic Your Way" pricing plan for admission, the difference between a 4-day pass and a 7-day pass is minimal. Heck, go for an 8-day pass and use it on your first or last day - at that point, it literally is only a couple of dollars more.
Use your best judgement - and decide what's more important to you and your mom. It would be a no-brainer for me, but everyone is different, as we wll know.
Good luck!
N.E.D.